Although some users have complained that the overall look and feel of iOS hasn't changed, that's a strength, not a flaw. iOS's success has much to do with its consistency; change for the sake of change often brings more confusion than it's worth. See: Windows 8.
I like the look and feel of iOS and I don't mind that, in general, the interface has remained consistent over the years. With that in mind, iOS 6 does offer a few UI refinements. The menu bar now adapts to the color scheme of whatever app or background is in use. (In the Phone app, for instance, the menu displays are a grayish-blue; in the App store, the same menu is black.) The iPhone Music app now sports a gray-themed interface that resembles the look of the iPad Music app in iOS 5. (The music player screen now shows off the darker theme, too.)
In addition to those changes, the iBooks, iTunes and App store apps get some updates: darker themes and home screens that now emphasize swipe gestures. Apple obviously worked for interface consistency across the board to make the stores easier to navigate.
Usage and final impressions
I snarkily refer to the annual period between WWDC and Apple's fall events as The Summer of Bugs, when Apple is ironing out the kinks in iOS. Surprisingly, this year was devoid of any big showstoppers. (I have noticed that some album art won't sync since upgrading, both on the iPad and the iPhone. Instead of high-resolution art, I'm seeing the default generic iTunes placeholder. Maybe it's solely a problem with my iTunes library, but it's something to note if you're an iTunes library perfectionist like me.)
Otherwise, iOS 6 is fast, stable and apparently free of major problems. So far, I've installed it on Retina display iPads, several iPhone 4Ses and an iPad 2. All took the manual updates without issue and ran the new OS without any performance degradation. Battery life has remained consistent as well.
As for my favorite changes, I love Photo Streams and the Camera app's Panorama mode; I've tweeted a couple of panoramas I took of some recent sunrises. Panorama mode makes taking great looking shots really easy. It's a shame it's not available on more models.
I'm also happy to see iCloud services working well, both as a backup/restore mechanism and as a way to sync my data across devices. There were many times throughout the summer I'd bump into a new feature, activate Siri and begin a "Note to self...." It was nice to sit down at my Mac and have all of those notes at my disposal, even though all of them were created on the iPhone. Thanks, iCloud.